1. Field of the Invention
The present invention relates to a terminal end processing method and a terminal end shielding structure of a shielded cable at a terminal end portion of a shield braid (shield layer), which shielded cable is used in transferring high-speed signals between electronic equipment, and to a light transmitting/receiving system using a terminal end shielding structure.
2. Description of the Related Art
In recent years, shielded cables, in which a bundle of core wires are covered by a net-like shielding member, have been used in order to reduce the unneeded radiation of electromagnetic waves at the time of carrying out high-speed signal transfer of from several tens of MHz to several GHz between electronic equipment.
In this shielded cable, a net-like end portion of the shielding member electrically contacts and is fixed to a metal, electrically-conductive case which is formed for electromagnetic shielding of electronic equipment. In this way, there can be obtained good effects of decreasing influence of the electromagnetic waves and reducing unneeded radiation.
For example, the following is proposed as a terminal end shielding structure of a conventional shielded cable: a supporting ring, which is formed in the shape of a hollow cylindrical tube, is placed on a shielding layer which is non-tin-plated. The shielding layer is folded back at the peripheral edge of the supporting ring, and an earthing wire also is made to contact the outer peripheral wall of the supporting ring. An electrically-conductive tube, which is formed in the shape of a hollow cylinder, is placed on the earthing wire and the shielding layer on the outer peripheral wall of the supporting ring. The terminal end shielding structure is crimped and fixed by applying a uniform pressure to the outer peripheral wall of the electrically-conductive tube from the outer side (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2001-286025).
As another terminal end shielding structure of a shielded cable, a ferrite compound layer is interposed between a shield braid and an electric insulating layer at the terminal end portion of a shielded cable. A toroidal core is attached to a separated portion of an insulating covering layer at the terminal end of the cable. The distal end portion of a shielding layer is folded back so as to cover the outer side of the toroidal core. An insulating cable is wound around the outer surface thereof. The distal end portion of the shielding layer is connected to a shielding metal cover, and forms a coil of one turn (see, for example, JP-A No. 2004-31291).
As yet another terminal end shielding structure of a shielded cable, there is proposed a structure in which the shield braid of a shielded cable is pressure-contacted and fixed by a clamp portion of an electrically-conductive inner side case, and is pressure-contacted and fixed also by a clamp portion of an electrically-conductive outer side case. In this way, a member of the outer side case member and the inner side case are reliably connected electrically, the mechanical strength of a fit-together portion is reinforced, and the electrical connection can be made stable (see, for example, JP-A No. 2002-117937).
In the above-described terminal end shielding structures of a shielded cable, during the work of folding-back the shield braid toward the outer skin at the end portion of the shielded cable, it is easy for the free end portion of the shield braid to be scattered and become unevenly distributed. Further, the gaps between the plural core wires which are wrapped by the shield braid collapse easily. It is therefore difficult for the cross-section of the end portion to be a stable circular configuration.
In the above-described conventional terminal end shielding structures of a shielded cable, when the braiding wires of the shield braid are disjoined and are folded back toward the outer skin, as described above, it is easy for the plural braiding wires to be distributed unevenly and randomly. Accordingly, the places of contact with the electrically-conductive member for shielding, which is disposed to as to envelop the outer peripheral surfaces thereof, are dispersed.
As a result, there are relatively few portions of contact with the case, and the impedance at the portions of electrical connection between the shield braid and an electrically-conductive tape is high. Therefore, when transferring high-speed signals of from several tens of MHz to several GHz, there is the problem that the effects of the countermeasures to EMI (electromagnetic interference) are insufficient.
Thus, in the above-described conventional shielding structures of a shielded cable, it has been thought to wind an electrically-conductive adhesive tape in order to gather together the respective wires which have been dispersed and increase the portions of electrical contact at the free end portions of the braiding wires of the shield braid.
However, in this case, the adhesive surface of the electrically-conductive adhesive tape abuts the outer peripheral surfaces of the free end portions of the braiding wires of the shield braid, and the contact resistance is great. Therefore, it is difficult to achieve an electrical connection at a low impedance. At the time of carrying out high-speed signal transfer of from several tens of MHz to several GHz, there is the problem that the effects of the countermeasures to EMI (electromagnetic interference) are insufficient.
In the case of a structure of nipping the portion at which portion the electrically-conductive adhesive tape is wound on the braiding wire free end portions of the shield braid as described above, by an electrically-conductive case for shielding which is divided into two parts and to be put together, there is dispersion in the thickness of the shield braid which is dispersed, and there is the possibility of insufficient contact at the restrained portion. Accordingly, when transferring high-speed signals of from several tens of MHz to several GHz, there is the problem that the effects of the countermeasures to EMI (electromagnetic interference) are insufficient.
In particular, in the case of a structure in which electrical continuity is achieved by nipping the free end portions of the braiding wires of the shield braid by an electrically-conductive case for shielding which is divided in two, if the configurations of the nip-in portions of these two cases form a diamond-shaped space for nipping by the combination of two triangular shapes, there are four points of contact. Accordingly, there is dispersion in the states of contact at the pressed-down portions.
In such a contact state, when high-speed signal transfer of from several tens of MHz to several GHz is carried out in order to stabilize the electrical connection, there is the problem that the effects of the countermeasures to EMI (electromagnetic interference) are insufficient.